Several applications require materials to be processed into a glassy, amorphous form. For this purpose, inorganic glasses (SiO2, for example), or polymers, are often employed, but small molecules are an appealing alternative because they are typically easier to purify, characterize and process due to the fact that they are monodisperse species. Small molecules capable of readily forming glassy phases at ambient temperatures are called molecular glasses or amorphous molecular materials, and currently see widespread use in optoelectronics (primarily as hole-transport materials in OLEDs), in nanolithography, and in amorphous drug formulations. [Hancock, B. C.; Zografi, G. “Characteristics and Significance of the Amorphous State in Pharmaceutical Systems”, J. Pharm. Sci. 1997, 86, 1-12. Shirota, Y. “Organic materials for electronic and optoelectronic devices”, J. Mater. Chem. 2000, 10, 1-25. Yu, L. “Amorphous pharmaceutical solids: preparation, characterization and stabilization”, Adv. Drug Deliv. Rev. 2001, 48, 27-42. Shirota, Y. “Photo- and electroactive amorphous molecular materials—molecular design, syntheses, reactions, properties, and applications”, J. Mater. Chem. 2005, 15, 75-93. Dai, J.; Chang, S. W.; Hamad, A.; Yang, D.; Felix, N.; Ober, C. K. “Molecular Glass Resists for High-Resolution Patterning”, Chem. Mater. 2006, 18, 3404-3411. Gra{hacek over (z)}ulevi{hacek over (c)}ius, J. V. “Charge-transporting polymers and molecular glasses for optoelectronic applications”, Polym. Adv. Technol. 2006, 17, 694-696.]
The two most commonly occurring problems with molecular glasses are: (1) limited accessibility of the glassy phase, as most compounds only form glasses when cooled extremely rapidly or through other special processing, and (2) their tendency to crystallize upon heating or standing for extended periods of time, due to the metastability of the glassy state and the higher mobility of small molecules relative to polymers. [Ediger, M. D.; Angell, C. A.; Nagel, S. R. “Supercooled Liquids and Glasses”, J. Phys. Chem. 1996, 100, 13200-13212.] Thus, the current challenge with molecular glasses is to design compounds capable of readily accessing the glassy state, even upon slow cooling, and that do not re-crystallize upon heating or prolonged standing. While several examples of such glasses have been reported, and some guidelines for molecular glass design have been established (for example, most glass-forming small molecules possess globular and irregular shapes to prevent efficient packing, and typically avoid strong and directional intermolecular interactions), the design of a glass-forming compound for a specific purpose requires some measure of trial-and-error screening of molecular structures, because the molecular structure must be tailor-made to fit the structural requirements for glass formation, often involving a multi-step synthesis where the molecular structure as a whole serves to disfavour crystallization. [Ishow, E.; Bellaïche, C.; Bouteiller, L.; Nakatani, K.; Delaire, J. A. “Versatile Synthesis of Small NLO-Active Molecules Forming Amorphous Materials with Spontaneous Second-Order NLO Response”, J. Am. Chem. Soc. 2003, 125, 15744-15745. Tamino, T.; Yoshikawa, S.; Ujike, T.; Nagahama, D.; Moriwaki, K.; Takahashi, T.; Kotani, Y.; Nakano, H.; Shirota, Y. “Creation of azobenzene-based photochromic amorphous molecular materials-synthesis, glass-forming properties and photochromic response”, J. Mater. Chem. 2007, 17, 4953-4963. Nagahama, D.; Nakano, H.; Shirota, Y. “Synthesis and Photochromic Response of a New Photochromic Amorphous Molecular Material Based on Spirooxazine”, J. Photopolym. Sci. Technol., 2008, 21, 755-757.] Therefore, there currently exists no general glass-inducing moiety that can be readily introduced by a simple synthetic procedure on a given compound to promote the formation of glassy phases.
Previously, we have developed a series of glasses based on mexylaminotriazines that show all the desirable properties for glass formation; in this case, however, it has been shown that hydrogen bonding contributes to promote glass formation through the formation of supramolecular aggregates that pack poorly. The hydrogen bonding provides an additional energetic barrier to reorganization of molecules in the solid state, which eventually leads to crystallization. [Wuest, J. D.; Lebel, O. “Anarchy in the solid state: structural dependence on glass-forming ability in triazine-based molecular glasses”, Tetrahedron, 2009, 65, 7393-7402. Wang, R.; Pellerin, C.; Lebel, O. “Role of Hydrogen Bonding in the Formation of Glasses by Small Molecules: A Triazine Case Study”, J. Mater. Chem., 2009, 19, 2747-2753. Plante, A.; Mauran, D.; Carvalho, S. P.; Page, J. Y. S. D.; Pellerin, C.; Lebel, O. “Tg and Rheological Properties of Triazine-Based Molecular Glasses: Incriminating Evidence Against Hydrogen Bonds”, J. Phys. Chem. B, 2009, 113, 14884-14891.]
We have previously demonstrated that compounds that readily crystallize, such as tetraphehylporphyrin (TPP), can be made to form glasses by functionalization with mexylaminotriazine units. However, in this example, it was necessary to build the glass-inducing moieties on the TPP core in several steps and a global yield close to 50%. [Meunier, A.; Lebel, O. “A Glass Forming Module for Organic Molecules Making Tetraphenylporphyrin Lose its Crystallinity”, Org. Lett., 2010, 12, 1896-1899.]
Therefore, novel molecular glasses that can be grafted covalently on a given core compound to induce the formation of glassy phases in one facile, high-yielding step is highly desirable, because it will (1) reduce the amount of screening necessary to identify structures which lead to a high propensity of forming glasses and a high longevity of the glassy state, and (2) enable to access molecular glasses with various properties for various applications in a rapid and efficient fashion starting from a few common precursor “snap-on” glasses that can be conveniently synthesized.